Ask Agaric: Where does Drupal store module dependency information?

"I can't see how drupal is storing the dependents. They are not in the system table info column."

Drupal doesn't store dependents, it gets them from the .info file each time:

http://api.drupal.org/api/function/drupal_parse_info_file/6
is called by
http://api.drupal.org/api/function/module_rebuild_cache/6
which is what also calls
http://api.drupal.org/api/function/_module_build_dependencies/6

Note: That is the real source of the dependency list, because module_rebuild_cache is also called by
http://api.drupal.org/api/function/system_modules/6
which is how
http://api.drupal.org/api/function/system_modules_submit/6
gets the list of modules in its form values that it then passes to
http://api.drupal.org/api/function/system_module_build_dependencies/6

The other key line from module_rebuild_cache is

<?php
$files = drupal_system_listing('\.module$', 'modules', 'name', 0);
?>

which gets the location of all modules, and hence info files.

Resolution